Aqueous coating compositions having a metallic effect have been known for many years. The metallic effect is achieved by means of lamellar metal pigments. In aqueous coating compositions, the corrosion-sensitive aluminum pigments must be provided with a protective coating in order to avoid oxidation of the aluminum pigments with the formation of hydrogen.
An important property of metallic coatings is their high luster. Luster is, inter alia, a physiologically and psychologically discerned variable, but according to DIN 67 530 the “luster capacity” of a plain surface can be determined by taking reflectometer readings. The reflection at the luster angle is measured relative to a standard (as a rule a pane of black plate glass). According to this standard, high gloss finished samples (reflectometer value >70) are measured at an angle of incidence or reflection of 20° and medium gloss finished surfaces are measured at 60°. A prerequisite for a good luster of metallic coatings is likewise optimal plane-parallel orientation of the lamellar pigments in the application medium.
The most brilliant aluminum pigments having the highest luster and flop are at present assigned to two classes: on the one hand so-called “silver dollar pigments”, which are produced by wet grinding of aluminum shot, and on the other hand so-called “PVD pigments”. Silver dollar pigments are distinguished from metal pigments obtained by comminution grinding by a relatively round shape and relatively smooth surface.
A coating composition which contains silver dollar aluminum pigments is described, for example, in DE 101 00 195 A1. Here, a neutralizing mixture of at least two fatty acids is present in the coating composition. After application, this coating composition is said to exhibit a markedly lower tendency to the formation of “clouds”.
Aluminum pigments having a high degree of reflection and high coverage produced by wet grinding are described, for example, in EP 0 451 785 B2. The pigments are characterized by degrees of water coverage (spreading value) of from 2.5 to 5.0 m2/g, a roughness value of 2.0 or less and an aspect ratio d50/h of 90 or more. From the examples of EP 0 451 785 B2, aspect ratios up to a maximum of 140 are known. No aqueous coating compositions, however, are described in said reference.
In WO 01/81483 A1, a pigment preparation which contains aluminum pigments covered with a silicon-oxygen matrix and a binder containing carboxyl groups having an acid number of from 10 to 100 mg of KOH/g is described. The thickness characteristics of the aluminum pigments used in this case are not described. Normally, however, pigments having layer thicknesses of over 300 nm had been on the market at that time.
In the case of PVD pigments, extremely thin aluminum pigments are produced (thicknesses: 20 to 50 nm). The thickness distribution of these pigments is extremely low. In this process, aluminum is evaporated in an ultrahigh vacuum onto a carrier foil provided with a release film (“release coat”). This release film is generally a polymer. Subsequently, the evaporated aluminum is separated, as far as possible, from the carrier film in a solvent and the metal foil is comminuted mechanically or by ultrasound. The production of PVD pigments is described, for example, in J. Seubert and A. Fetz, “PVD Aluminum Pigments: Superior Brilliance for Coatings and Graphic Arts”, Coatings Journal, Vol. 84, A6 225-264, July 2001, pages 240-245.
These PVD pigments have outstanding covering power on account of their extreme thinness. The thin pigments are so flexible that they really “cling” to their substrate. They should therefore be applied to a smooth substrate to display their visual possibilities.
Coating compositions containing PVD pigments in passivated form make it possible to produce coatings having a higher luster and the visually perceptible appearance of a cohesive metal film. However, on account of the expensive PVD pigments, coating compositions of this type are likewise correspondingly expensive.
Furthermore, it is drawback that the release coat can scarcely be completely removed from the pigment particles. This adhering polymer film, however, can be troublesome. This is manifested, for example, by the formation of agglomerates, which completely ruin the desired decorative effect. Agglomerates of this type lead to spot formation and increase the tendency to cloud formation.
However, polymeric adhesions of this type can have a particularly detrimental effect when the aluminum pigments are provided, after their formation, with chemical protective coatings in order to make them corrosion-resistant, such as are described in DE 196 35 085.
A further disadvantage of PVD pigments of this type is that they are difficult to handle in aqueous coating compositions. On account of their extremely high and smooth specific surface area the tendency to agglomerate is very high. In addition, coating compositions which contain these pigments are relatively difficult to apply. Automobile coatings having a uniform appearance are achieved, for example, only by means of hand coating in many thin-layer coating steps. Sensitive application behavior of this type stands in the way of automation, as is customarily employed for coating compositions containing conventional aluminum pigments.
Aqueous coating compositions which contain conventionally produced pigments—in the form of silver dollars or “cornflakes”—in passivated form, are suitable for producing metallic coatings.
The effects achievable thereby exhibit a certain metallic luster and flop. The visually perceptible appearance, however, is always such that a certain granularity of the coating can be seen. Also, on account of the thickness of these conventional pigments (>300 nm), relatively high pigmentation levels must be chosen in order to guarantee complete coverage.
In addition, many aqueous coating compositions containing aluminum pigments are prone, after application, in particular by spraying, to the formation of so-called “clouds”. This is manifested by the formation of varying light and dark shades in the coating and has an extremely objectionable influence on the effect produced by the effect coating.
Cloud formation of this type is particularly objectionable if the metallic coating per se already conveys an extremely high-quality impression, i.e. is associated with high luster and flop.
The formation of clouds takes place as a result of an irregular orientation of the aluminum pigments, in particular as a result of aluminum pigments standing virtually perpendicularly in the coating.